The present invention relates to a catalytic converter utilizing an intumescent mounting mat for mounting a ceramic monolith within a metallic casing to produce a catalytic converter.
Catalytic converters are universally employed for oxidation of carbon monoxide and hydrocarbons and reduction of the oxides of nitrogen in automobile exhaust gases in order to control atomospheric pollution. Due to the relatively high temperatures encountered in these catalytic processes, ceramics have been the natural choice for catalyst supports. Particularly useful supports are provided by ceramic honeycomb structures as described, for example, in U.S. Patent Re 27,747. These ceramic bodies tend to be frangible and have coefficients of thermal expansion differing markedly from the metal, usually stainless steel, casings of the converters. Thus, the mounting means for the ceramic body in the container must provide resistance to mechanical shock due to impact and vibration. Intumescent sheet materials useful as mounting materials for this purpose are described in U.S. Patents 3,916,057, 4,305,992 and 4,617,176 and in U.K. Patent 1,513,808.
Catalytic converters employing intumescent sheet mounting materials generally use a single rectangularly shaped sheet or mat usually having a slot on one end and a complementarily configured tab on the other. The sheet or mat is wrapped around the lateral (outer) peripheral surface of the ceramic monolith with tab and slot engaged. The catalytic converter of U.S. Pat. No. 4,617,176 uses a generally rectangular shaped mat with sinusoidal lengthwise edges to minimize axial stresses in the ceramic monolith but still employs the tab and slot geometry. A disadvantage of such systems is that the tab must fit exactly into the slot to ensure a tight gas seal about the preiphery of the ceramic monolith. Unfortunately, this is difficult to do in an automated process and hand operations are expensive and time consuming.
In addition, since the ceramic monolith has a very large dimensional tolerance (typically .+-.2mm diameter), the circumference or perimeter of the monolith can vary more than .+-.6 mm. This means that for a given mat length, the mat will tend to be either too long or too short as the individual ceramic monolith size varies within its tolerance range, fitting exactly only a monolith with an exactly nominal perimeter. Since mat overlap can cause improper shell closure with the attendant risk of mounting failure or crushing of the ceramic monolith, overlap is usually avoided by designing the mat length for a monolith of the maximum perimeter. This ensures that mat overlap will never occur, but also means that the mat will be too short in many cases, leaving a space where the two ends of the mat should come together. This space is undesirable because it provides additional mat edge area for the exhaust gas to impinge upon. Under severe driving conditions this can provide a site for mat erosion to begin. This space also allows more heat to be conducted to the metal shell since hot exhaust gases are in direct contact with the metal shell.
The large tolerance of the ceramic monolith also affects the mount of packing density of the intumescent material. With presently available materials, a minimum mount density of about 0.64 g/cm.sup.3 is required to hold the ceramic monolith in place under normal conditions and 0.75 g/cm.sup.3 under extreme conditions. The mount density must be kept below about 1.36 g/cm.sup.3 for a typical 62 cell/cm.sup.2 monolith or monolith breakage can occur either during assembly or in operation. In order to achieve and maintain this mount density range under severe operating conditions, a single mat as thick as 9.8 mm has been used.